The method of activating a vehicle fuel cell improves performance of a polymer electrolyte membrane fuel cell, to ensure maximum performance, reduces hydrogen usage, and stabilizes performance after manufacture.
In general, a fuel cell includes an electrode in which an electrochemical reaction occurs, an electrolyte membrane which transfers hydrogen ions generated by the electrochemical reaction, and a separator which supports the electrode and the electrolyte membrane.
Recently, polymer electrolyte membrane fuel cells have been introduced for use as vehicle fuel cells. In comparison with other types of fuel cells, the polymer electrolyte membrane fuel cell has an excellent efficiency, a high current density, a high output density, and a short start-up time. In addition, since a solid electrolyte is used, corrosion and electrolyte control are not necessary in such polymer electrolyte membrane fuel cells. Furthermore, the polymer electrolyte membrane fuel cell is an environmentally friendly power source in which only pure water is discharged as an exhaust gas. Therefore, the polymer electrolyte membrane fuel cell is currently being researched worldwide in the automobile industry.
The polymer electrolyte membrane fuel cell generates water and heat through an electrochemical reaction between hydrogen and oxygen. Supplied hydrogen is decomposed into hydrogen ions and electrons by a catalyst in an anode electrode. The decomposed hydrogen ion is transferred to a cathode electrode through an electrolyte membrane, and is combined with supplied oxygen and the electrons transferred through an external conductive wire to generate water, thereby generating electronic energy.
In this case, an ideal electrical potential is about 1.3V, and can be expressed as a following chemical reaction equation.Anode: H2→2H++2e Cathode: ½O2+2H++2e→H2O
In practice, a vehicle fuel cell requires an electrical potential higher than the above electrical potential. To obtain a higher electrical potential, individual unit cells have to be laminated together until a desired electrical potential is achieved. The unit of laminated cells is referred to as a stack.
A fuel cell electrode is made by combining a hydrogen ion carrier such as nafion with a catalyst such as platinum. If an electrochemical reaction occurs when a fuel cell is initially driven after the fuel cell is manufactured, the fuel cell becomes less activated. This is because a reactant cannot reach the catalyst since a reactant passage is blocked, the hydrogen ion carrier such as the nafion, which forms a triple phase boundary with the catalyst, is not easily hydrated at an initial driving stage, and a continuous movement of hydrogen ion and electrons is not ensured. For these reasons, an activation process is required so as to ensure a maximum performance of a fuel cell after the fuel cell is assembled using a membrane electrode assembly and a separator.
The purpose of the activation process is to activate a non-reactive catalyst and to sufficiently hydrate an electrolyte included in an electrolyte membrane and an electrode, thereby ensuring a hydrogen ion passage. The activation process is carried out so that the fuel cell can reach its maximum performance after assembly. This may take a number of hours or days based on driving conditions. When the activation process is not properly performed, the fuel cell may not operate at its maximum performance.
Fuel cell manufacturers have their own methods of activating a fuel cell. In a typical activation method, the fuel cell is driven for a long time under a specific voltage. In the conventional method of activating a fuel cell, the fuel cell is exposed to a low voltage for a long time, so that the fuel cell can be activated even in a portion where its stack performance is no longer improved.
Disadvantages of the conventional method lie in that time efficiency decreases since it takes a long time for a fuel cell to reach its maximum performance. Furthermore, it takes a long time for the fuel cell to be completely manufactured after a stack is manufactured. In addition, excessive hydrogen and air are consumed while the fuel cell is driven, thereby decreasing price competitiveness.